This invention relates to signal processing. Specifically, the present invention relates to encoding and decoding audio signals for driving multiple speakers.
Many systems have been developed to reproduce three-dimensional sound effects using multiple loudspeakers. One of the most sophisticated systems is the Ambisonic method of recording information about a sound field for reproduction over some form of loudspeaker array so as to produce the impression of hearing a true three-dimensional sound image.
The Ambisonic system encodes a three-dimensional soundfield in a B-format which is defined as follows:
X=cos(Az) (front-back velocity signal)
Y=sin(Az) (left-right velocity signal)
W=0.707 (pressure signal)
where the vertical dimension has been ignored and Az is the azimuth (horizontal panning angle) of the sound source.
Note that the Y component can be positive or negative which requires a signal processing unit which can generate negative values. However, some systems are capable of generating only positive loudspeaker weights and are unable to implement B format encoding.
According to one aspect of the present invention, a method and system for panning sounds over four speakers employs a three-channel representation of the same. With the present invention, panning sounds over four speakers (denoted L,R,SL,SR) may be achieved while employing a three-channel representation of channels (L,R,W). However, (L,R,W) is more convenient than (W,X,Y) because it already contains the signals for the two front speakers.
When the three-channels are converted to four speaker feeds, it is ensured that positive loudspeaker weights are produced instead of out of phase feeds in the loudspeakers opposite the direction from where the sound is panned, as is done using the Ambisonic prior art technique.
According to another aspect of the invention, this is achieved by using a sine-cosine panning technique to pan over a pair of left and right channels, while including the unprocessed input signal (W) in this three-channel form at. A subsequent operation then allows deriving the two rear channel signals for reproduction over four speakers. In this fashion, the three-channel format (L,R,W) could be decoded over any horizontal multi-channel loudspeaker setup.
According to one aspect of the invention, panning over four loudspeakers is implemented utilizing an internal representation having three channels.
According to another aspect of the invention, a signal including an audio portion and angular information is encoded in Left, Right, and W channels where all channel weights have positive values.
According to another aspect of the invention, sine/cosine panning, using the raised cosine function, is utilized to calculate Left and Right speaker weights and the W channel is equal to the input audio signal.
According to another aspect of the invention, the Left and Right channels are used directly to drive front loudspeakers while the other loudspeakers are driven by combinations of the Left, Right, and W channel.
The use of only positive weights in the three-channel format is convenient for hardware designers because it is easier to design hardware to implement positive weights. Also, subsequent to decoding, the positive weights allow the panning of signals from front left to right (or vice versa) with no signals sent to the rear speakers, which results in more realistic effects.